1. Field of the Invention
The present invention relates to a mechanism of an eyepiece for an endoscope. More particularly, the present invention pertains to a mechanism of an eyepiece for an endoscope which is designed so that an ocular lens is movable along the optical axis for the purpose of dioptric adjustment and, when an attachment of a photographic recording device or an imaging device is attached to the eyepiece, the ocular lens is automatically moved to a predetermined projection position within the range of dioptric adjustment, no matter where the ocular lens is disposed in the dioptric adjustment range at that time.
2. Description of the Related Art
The eyepiece of an endoscope that is in general use is designed so that an ocular lens is not only employed for observation by the naked eye but also used as a projection lens for photographic recording or other imaging purposes. More specifically, the ocular lens is adapted to be movable along the optical axis within the range of from a minus diopter position to a plus diopter position so that, when the ocular lens is employed for observation by the naked eye, the level of diopter is adjustable to the user's eye.
There has heretofore been one type of eyepiece structure for endoscopes in which the ocular lens, when used as a projection lens, projects the real image. From the viewpoint of the degree of freedom with which the system is handled and also the realization of a waterproof structure, however, it is more advantageous to set the ocular lens at the zero diopter position so as to project the image at infinity. For this reason, it has been common practice to move the ocular lens to the zero diopter position, halfway in the dioptric adjustment range, when it is used as a projection lens for photographic recording or other imaging purposes.
In eyepiece mechanisms for endoscopes proposed in the early stage of development of the art, the position of the ocular lens is adjusted by turning a dioptric adjustment ring by a manual operation every time the ocular lens needs to be moved to the zero diopter position. With this arrangement, however, the operation is complicated and troublesome and, every time the system is to be used for observation by the naked eye after the use for photographic recording or other imaging purposes, dioptric adjustment must be made over again. Accordingly, more advanced eyepiece mechanisms are provided with a lens moving means that is adapted to move the ocular lens to the zero diopter position in response the attaching of a photographic recording device or an imaging device to the eyepiece, thereby enabling the ocular lens to be automatically moved between the projection position and an appropriate diopter position in linkage with attaching and removal of the attachment.
In this prior art, setting of the ocular lens to the projection position is effected by pressing it against a cam surface by means of the biasing force of a spring which is applied to the lens in the direction of the optical axis. Therefore, two cams are provided one cam is adapted to push the ocular lens back to the projection position when it is at a forward position (i.e., a plus diopter position) of the projection position (i.e., the zero diopter a second cam is adapted to push the ocular lens back to the projection position when it is at a rearward position (i.e., a minus diopter position) of the projection position.
Setting of the ocular lens to the projection position must be effected with a considerably high degree of accuracy. This is because even a very small error in the lens position, for example, on the order of 0.02 mm, has a critical effect on the projection of the image and makes it impossible to obtain an in-focus picture.
In this regard, the above-described prior art suffers from several problems. The prior art has two cam surfaces for setting the ocular lens to the projection position and these two cam surfaces are alternatively employed according to whether the ocular lens is at a plus diopter position or a minus diopter position as a result of the preceding dioptric adjustment. It is extremely difficult to form two cam surfaces at exactly the same position due to the limited degree of accuracy in machining. Accordingly, if focusing is effected on the basis of one cam surface, there may be an error in the focus when the other cam surface is used to set the ocular lens to the projection position. In such a case, it is impossible to obtain a clear picture.